New install, low frequency distortion at low volume

I just installed a set of four Infinity Kappas and a 4 channel Infinity Reference amp into my 2000 Honda Insight.

I started by just replacing the door speakers and leaving them powered by the head unit just to hear the difference, and frankly the Infinity Kappas were a phenomenal upgrade over the Sony Garbage? that was there. I found I was hearing detail that was previously entirely lost, and was able to flatten the equalizer.

Next, I replaced the rear speakers. I had to get out my jigsaw and make some "modifications" because of the depth. From these I ran 12 gauge braided copper to the amp, a run of about 3-4ft tops. I then ran the ground and 12v lines to a new amp (Infinity Reference) with 10 gauge. The 12v line runs directly from the battery (fused) through a grommet in the firewall. The ground is next to the amp, under the passenger seat.

After connecting the RCA cables from my head unit, I immediately noticed a lot of hiss and high pitched whine which wasn't present without them plugged in. Turning on some music, I noted the rear speakers sounded like complete garbage. Aside from the whine, low frequencies were completely garbled, worse than crossed phases, more like blown-speaker sounding. Low frequencies (below maybe 350hz) are severely distorted. I'm not sure if I'd call it clipping, but it might be.

-I tried playing with gain, HPF and LPF on the amps. The distortion is present at low volumes even with minimum gain.

-I pulled the Infinity amp out and put an older Kenwood class AB amp I have in, and while the whine wasn't as loud, low frequencies still sounded garbled and awful, so not the amp?

-I tried switching the phases to the speakers, which made no difference.

-I tried using both front and rear outputs from the head unit, and both front and rear inputs on both amps.

-I tried powering the amp through the 12v power socket (different +12v source and ground) and that made no difference.

-I pulled the dash apart and stretched out my RCA cables, which were really too long by a lot (10ft?), and frankly are garbage cables. With them away from everything else (EMF) and not bundled up, the whine was reduced on the Infinity amp but the low frequencies did not improve and still sounded blown out. I have on order a pair of correct-length shielded beefy RCA cables, but I doubt that will solve this.

-I cut and spliced the RCA cables down to a very manageable ~24 inches, no improvement.

-I tried connecting the amps to my phone as a source, rather than the head unit, and still the lows sounded bad.

So, I believe I've ruled out a bad amp since they both do it. I've ruled out head unit as a source of garbled audio by using my phone as source. I've used different cables and swapped phases. I've tried a different power source. I have not yet tried jumping the amp by shorting the remote to +12v, and running it from battery while the car is off (so no electronics possibly interfering), or using the old speakers, because pulling the rear deck apart is a real chore. The Infinity speaker box was seemingly unopened, and both left and right speakers sound the same, which makes me inclined to believe they aren't both blown new speakers from the factory in the same box.

Everything below maybe 350hz is just horribly distorted and garbled. The rears sound alright when I set a high pass filter to around 350hz.

I think my approach when I get home from work today will be to power the amp with the car off, try using my phone as source, and try both new and old speakers to see if perhaps the both of the pair of Kappas I got were defective. Any troubleshooting advice is welcomed.

EDIT: Corrected wire size used.

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